Semiconductor chips are continually increasing in power, compactness and waste heat production. While they must be consequently cooled, for a number of reasons, this is generally not done by directly cooling the chip itself with a forced stream of cooling fluid or liquid. At least one surface of a chip is exposed for heat extraction, although it generally is not directly exposed, itself being covered by a thin “lid” of polished metal, sometimes referred to as an integral heat sink. The exposed surface area of the chip or CPU is small, and in order to forcibly contact such a small surface with a sufficient and effective volume of coolant (whether air or liquid), the flow rate would be high. Consequently, the exposed surface of the chip is generally thermally bonded (by a suitable conductive glue like material) to the flat lower surface of a conductive metal plate, sometimes called a cold plate or a cold sheet, although often described by the misnomer “heat sink.” The plate typically is regular and rectangular in cross section, with a flat lower surface significantly larger in area than the chip, and an upper surface of equal area that comprises a series of straight, parallel, thin and closely packed fins. Heat does not “sink” or disappear in the plate in any sense, but is continually removed therefrom as it flows into the plate from the chip being cooled. Consequently, the plate may be more accurately referred to as a “spreader plate,” that is, a plate designed to spread out the area from which heat can be extracted. The upper surface (and fins, if present) are directly exposed to the forced fluid to convectively remove heat from the chip, which is effectively protected.
The essentially universal shape for the conductive plate, hereinafter referred to as a spreader plate, is a flat, constant thickness sheet, with parallel upper and lower, equal area flat surfaces. The heat flux in this or any other conductor, that is, the heat flow per unit of cross sectional area, is uniquely determined by the nature of, the shape and conductivity of, the medium itself, and is driven by a temperature gradient, that is, the difference in temperature between the lower surface (where heat is added from component 10) and the upper surface, where heat is extracted, generally by a forced air or liquid flow. A typical shape of such a plate is shown in FIGS. 1 and 2 of U.S. Pat. No. 3,361,165, and also in FIG. 1 of the drawings of the instant application, with a component 10 bonded to the larger bottom surface of a “heat sink member” 12, a conventional flat plate shape, rectangular in cross section. The cooling scheme disclosed in the patent is somewhat atypical, in that the plate 12 itself contains internal liquid channels for cooling, rather than fins on the upper surface of the plate cooled by pumped air or liquid. This particular patent does recognize that the heat flux in such a design is not symmetrical and regular, but is instead concentrated and denser near the center, directly over the component 10. This results from the fact that the central area of the flat upper surface is closer to the lower surface than the peripheral area. This asymmetrical flux density is represented by the curved but essentially vertical dotted lines, sometimes referred to in the literature as “adiabats” and the isothermal lines or isotherms, the curved lines drawn perpendicularly to the heat flux lines. Together, the crossing lines create a checkered series of cross sectional area elements. The approach described in the patent itself is not to modify the degree of thermal asymmetry, but rather to concentrate the fluid carrying channels near the center, or the area of highest heat flux. The lower corners especially of the plate, the corners of the rectangular cross section, provide little assistance in the heat removal process, as they contain no channels, and are essentially wasted mass.
As noted, it is more common to simply supply a heat transfer fluid or gas to the upper surface of such a spreader plate, which is often enhanced with added fins or the like, rather than to create internal channels in the plate. Unless the cooling fluid/gas applied to the upper surface of such planar spreader plate can be concentrated toward the center, or unless physical heat transfer enhancements, such as pin projections, are more densely applied to the central area, the heat transfer out of the spreader plate will be less efficient. Either alternative is more costly and difficult than simply applying the cooling fluid and/or any heat transfer enhancements symmetrically and evenly to the spreader plate upper surface.